Digesting apparatus



Dec. 7, 1965 D. J. DONALDSON DIGESTING APPARATUS Original Filed March25. 1960 INVENTOR. DONALD J DONALDSON ATTORNEY United States Patent3,222,141 DIGESTING APPARATUS Donald J. Donaldson, Baton Rouge, La.,assignor to Kaiser Aluminum & ChemicalCorporation, Oakland, Calif., acorporation of Delaware Original application Mar. 23, 1960, Ser. No.17,177, now Patent No. 3,112,994, dated Dec, 3, 1963. Divided and thisapplication July 9, 1962, Ser. No. 214,155 2 Claims. (Cl. 23-290.5)

This application is a division of Serial No. 17,177 filed March 23,1960, now Patent No. 3,112,994.

This invention relates to digesting methods and apparatus and moreparticularly to a novel method and apparatus for digesting aluminousores in caustic alumiuate liquors.

In the classic Bayer process for producing alumina from aluminous ores,a digestion step is performed to extract and solubilize the alumina fromthe raw aluminous ores in caustic aqueous aluminate liquor. This isgenerally accomplished at elevated temperatures and pressures inpressure digesters. These digesters comprise large, cylindrical,horizontally disposed vessels into which a slurry of the aluminous oreand caustic alumiuate liquor is pumped. Live steam at high pressures isusually introduced into the slurry and/or conventional indirect heatexchangers are used to heat the slurry or caustic aqueous alumiuateliquor to the desired digestion temperature.

The slurry mixture is maintained within the digester for a predeterminedperiod of time to permit extraction of the alumina and solubilization inthe liquor, after which time the slurry mixture is flash cooled andpumped out and then treated according to the well-known subsequent stepsof the Bayer process.

The above-described digester Vessel produces some degree of extractionof the alumina. However, due to the design of the digester vessel andthe manner in which it is used, short-circniting of the slurry usuallytakes place through the vessel. This short-circuiting of the slurrythrough the vessel reduces the over-all efiiciency of the extractionstep.

It is a prime purpose of the instant invention to provide an improvedmethod and apparatus for continuously digesting aluminous ores incaustic alumiuate liquor whereby maximum efficiency in the process ofextracting and solubilizing aluminous ores in caustic aluminate liquorsis attained.

Other advantages and purposes of the instant invention will be observedby reference to the following detailed description when taken inconjunction with the appended drawings, which discloses a preferred formof apparatus for carrying out this invention, wherein FIG. 1 is avertical cross-sectional view of one type of vessel that may be used inpracticing the instant invention;

FIG. 2 is a sectional view taken along line 22 of FIG. 1; and

FIG. 3 is a sectional view with parts removed taken along line 33 ofFIG. 2.

Referring now to the drawings, the digester of FIG. 1 generallycomprises a vertical cylindrical shell or vessel preferably made of amaterial resistant to the chemical action of reactive materials andstrong enough to withstand high temperatures and pressures. Securelyattached to the inner wall of vessel 10 in horizontally disposed andspaced relationship are a plurality of doughnut shaped baflle rings 12.Each bafile ring is of a suitable thickness and extends inwardly a shortdistance from the inner Wall whereby a large aperture 13 is provided ineach ring 12 through which extends a rotatable shaft 14. This shaft isrotated by any suitable means such as by motor 13 mounted on platform14' through the medium of any 3,222,141 Patented Dec. 7, 1965conventional power transmitting means such as gears or the like as at 5.The shaft 14 is supported by sealed bearing members 18 and 19 so thathigh pressure operations may be conducted within the vessel. A pluralityof circular baffle plates 15 are secured to shaft 14.

In an advantageous embodiment of the invention, a plurality of baflleplates 15 are positioned along the axial length of shaft 14 in spacedrelationship to each other and each of these plates is provded with aseries of vertically disposed stirring paddles 16. Although thesepaddles are shown as being curved, they may also be flat or straight invertical cross section and, in any event, their major surface areas 16'are disposed in vertical planes so that they cannot violently agitatethe contents of the vessel. Plates 15 can be advantageously split intotwo segments 17 and 17'. These segments are secured to each other bymeans of conventional split collars 20 and bolts 21. Each split collar20 is secured to its respective plate segment 17 or 17 by bolts 22. Eachcomplete plate 15 is keyed to the shaft 14 by means of the split collars20 and the key 23. Key 23 is disposed in recess 24 in one of the splitcollars 20 and in cooperating recess 25 in the shaft 14.

Screws 26 are finally inserted in suitable openings in one of the splitcollars 20 and the shaft 14 and complete the anchoring of the plates 15along with the blades or paddles 16 to the shaft. Paddles 16 are securedto the plate segments 17 or 17 by bolts 27. Additional bolts 28 can beused to secure the segments 17 and 17 to each other. Each plate 15 isadvantageously disposed substantially midway between a pair of bafflerings 12. The vessel 10 is provided at the top portion thereof withinlet means such as line 30 and at the bottom thereof with outlet meanssuch as line 31.

During an operating cycle, a slurry of aluminous ore in causticaluminate liquor is pumped into the vessel 10 through line 30. Theliquor has been heated to very high temperatures by live steam injectionand a series of heat exchangers. The liquor flows slowly through thevessel and a predetermined nominal or average holding time or residencetime is maintained by regulating the flow rates. As the liquor flowsthrough the vessel, the motor 13' slowly rotates the shaft 14 therebycausing the paddles 16 to merely gently stir theliquor without producingany violent agitation of the same. The arrangement of baflle rings 12and rotating plates 15 and paddles 16 with respect to each other i suchthat short-circuiting and back-mixing of the contents of the vessel areminimized and the flow characteristics of the contents of the vesselapproximate a plug-type flow. In other words, as the vessel iscontinuously filled with a slurry of aluminous ore in caustic alumiuateliquor, the slurry can be considered as being comprised of individualsuccessive slurry layers stacked one on top of each other. The additionof a new layer at the top of the vessel produces a correspondingdisplacement of each succeeding lower layer formed between adjacentbafile rings 12. Because of the disposition of baffle plates 15 andpaddles 16 between baffle rings 12, their particular configuration andslow speed of travel, no violent agitation of the slurry occurs in thelayers or compartments formed between bafile rings 12. Instead, thedisposition and structure of baflle rings 12, baffie plates 15 andpaddles 16 are such that they merely direct the flow of liquor andaluminous ores through the vessel while at the same time minimizingshort-circuiting and vertical mixing within the vessel. These elementsalso cooperate with one another in such a fashion that a circular motionwith a minimum amount of vertical flow is imparted to the contents ofthe vessel as they pass through the vessel. Accordingly, although thevarious aluminous ore particles in the slurry follow a roughly helicalpath of travel through the vessel, this helical path of travel Aconsists of relatively wide, individual turns or windings spaced a shortdistance apart with the diameter of the windings being substantiallygreater than the pitch of the windings. The bafile rings 12 alsominimize back-mixing of the slurry. Thus, the vessel actually comprisesa series of separate compartments or zones in which the flow of theslurry is directed or controlled to minimize vertical flow andshort-circuiting. Each compartment thus approximates an individualplug-type flow reactor zone.

In one specific example of apparatus and method which has been used todate to extract alumina from primarily monohydrate-type ores, threeidentical vertically arranged vessels disposed and operating in serieswere used. Each vessel had a length of 8'94 i and an internal diameterof 11'6". Each vessel was divided into eight compartments separated bydoughnut shaped baflle rings 12 spaced 9 apart. Each of these rings hadan internal diameter of 9'6 whereby an orifice of 9'6" existed betweeneach of the compartments 40 formed by rings 12. A flat baffle plate 1528%" in diameter was mounted on shaft 14 and centrally disposed in eachcompartment. Each plate 15 was located substantially midway between andin staggered relationship to a pair of rings 12 and the various adjacentplates 15 were spaced 9 apart. Shaft 14 was rotated at approximately 40rpm. There was continuously introduced into the first vessel throughline 30 an aqueous stream comprised of a bauxite slurry made up of 50%solids with spent caustic liquor and containing primarilymonohydrate-type aluminous ores. This stream had been previously blendedwith an excess amount of spent liquor containing 210 g./l. caustic inaccordance with the usual good practice in this art. This latter liquorwas previously heated to 4l5430 F. in a series of heat exchangers. Livesteam at 650 p.s.i.g. was injected into the above stream which flowed inat a rate of 6.072 gallons per minute to maintain a temperature of about473 F. and a pressure of 540 p.s.i.g. in the digester. The stream waswithdrawn from the first vessel at a rate of 6,200 gallons per minuteand then fed to the next succeeding vessel in the series where thestream was again treated as described above. Thereafter, the treatedstream was removed from the second vessel or digester and fed to thethird and final vessel or digester where the stream was again treated inthe same fashion as in the two preceding vessels. The mean residencetime for any given solids particle in each vessel was approximately tenminutes or approximately thirty minutes total mean or average residencetime in the series of vessels. It i to be understood, of course, thatdepending on the type of ores being treated and the specific resultsdesired the aforesaid residence times can be changed as required.

Based on this total residence time, it was found that approximately 99%of the total available alumina in the ore was extracted. This residencetime was obtained during passage of the liquor and ore through thesuccessive separate reactor zones or compartments 40 in each vessel byretarding the flow of liquor and ore through the disclosed anddescribed.

zones. This retarding was effected by gently and intermittentlyconverting the normal laminar flow of liquor and ore to a slowcirculatory flow and without producing any violent stirring andagitation of the liquor and ore. The use of baflle rings 12 togetherwith bafile plates 15 contributed materially in maintaining thedesirable total residence time of approximately thirty minutes bycontrolling and producing a plug-type How of the stream comprised ofliquor and aluminous ores through the vessel and in minimizingshort-circuiting. It will be understood that the above specificdescription of operating conditions sets forth one example of the modeof carrying out the invention. Where caustic i referred to in thisspecification and claims, it is intended to mean NaOH or caustic sodacalculated as Na CO An advantageous embodiment of the invention has beenIt is obvious that various changes and modifications may be made thereinwithout departing from the spirit and scope thereof as defined by theappended claims.

What is claimed is:

1. An apparatus for continuously digesting an aluminous ore in a causticaluminate liquor under conditions of high temperatures and pressures,comprising an elongated vertically disposed vessel, a plurality ofhorizontally disposed and spaced bafile rings extending inwardly a shortdistance from the inner wall of said vessel and forming a plurality ofsuccessive intercommunicating and separate unobstructed reactorcompartments in vertical stacked relationship, an elongated shaftconcentrically disposed within said rings and projecting through saidcompartments, a plurality of horizontally disposed baffle plates havingpaddles afiixed thereto and projecting outwardly therefrom with themajor surface areas of said paddles arranged in vertical planes, saidbaffle plates being mounted on said shaft in spaced relationship to eachother such that a baffle plate is disposed in each reactor compartmentintermediate a pair of bafile rings, said bafile plates beingconcentrically disposed on said shaft and within said baffle rings, theoutside diameter of said bafile plates with paddles being less than theinside diameter of said bafiie rings, means for slowly rotating saidshaft and the bafile plates whereby a circular motion with a minimumamount of vertical flow and agitation is imparted to the contents of thevessel, and inlet and outlet means at opposing end portions of thevessel.

2. An apparatus as set forth in claim 1 wherein each baflle plate isdisposed midway between a pair of bafile rings.

References Cited by the Examiner UNITED STATES PATENTS 2,601,674 6/1952Reman 23270.5 2,850,362 9/1958 Scheibel 23270.5 2,926,182 2/1960 Sutton23-285 X MORRIS O. WOLK, Primary Examiner. JAMES H. TAYMAN, 1a.,Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,222,141 December 7, 1965 Donald J. Donaldson It is hereby certifiedthat error appears in the above numbered patent requiring correction andthat the said Letters Patent should read as corrected below.

Column 1, line 43, for "prime" read primary line 52, for "discloses"read disclose line 55, b f "vessel" insert digester column 2, line 9,for "provded" read provided column 3, line 37, for

"6 072" read 6 ,072

Signed and sealed this 1st day of August 1967.

(SEAL) Attest:

Edward M. Fletcher, Jr.

EDWARD J. BRENNER Attesting Officer Commissioner of Patents

1. AN APPARATUS FOR CONTINUOUSLY DIGESTING AN ALUMINOUS ORE IN A CAUSTICALUMINATE LIQUOR UNDER CONDITIONS OF HIGH TEMPERATURES AND PRESSURES,COMPRISING AN ELONGATED VERTICALLY DISPOSED VESSEL, A PLURALITY OFHORIZONTALLY DISPOSED AND SPACED BAFFLE RINGS EXTENDING INWARDLY A SHORTDISTANCE FROM THE INNER WALL OF SAID VESSEL AND FORMING A PLURALITY OFSUCCESSIVE INTERCOMMUNICATING AND SEPARATE UNOBSTRUCTED RECTORCOMPARTMENTS IN VERTICAL STACKED RELATIONSHIP, AND ELONGATED SHAFTCONCENTRICALLY DISPOSED WITHIN SAID RINGS AND PROJECTING THROUGH SAIDCOMPARTMENTS A PLURALITY OF HORIZONTALLY DISPOSED BAFFLE PLATES HAVINGPADDLES AFFIXED THERETO AND PROJECTING OUTWARDLY THEREFROM WITH THEMAJOR SURFACE AREAS OF SAID PADDLES ARRANGED IN VERTICAL PLANES, SAIDBAFFLE PLATES